lt3724 Linear Technology Corporation, lt3724 Datasheet - Page 16

• Manufacturer Part Number: lt3724
• Description: High Voltage, Current Mode Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

LT3724
APPLICATIO S I FOR ATIO
The desired soft-start rise time (t
programming capacitor C
sponds to 2μA average current during the soft-start inter-
val. This capacitor value follows the relation:
R
Considerations for Low-Voltage Output Applications
The LT3724 C
cycle, and this voltage is increased at Figure 4 node “A” by
the 2μA signal current through R
reach this value before the soft-start function is engaged.
The value of this output soft-start startup voltage offset
(V
Which is typically 0.64V for R
In some low voltage output applications, it may be desir-
able to reduce the value of this soft-start startup voltage
offset. This is possible by reducing the value of R
reduced values of R
voltage ripple on the output must be minimized for proper
soft-start operation.
Peak-to-peak output voltage ripple (ΔV
posed on node “A” through the capacitor C
of R
It is important to use low ESR output capacitors for
LT3724 voltage converter designs to minimize this ripple
voltage component. A design with an excessive ripple
component can be evidenced by observing the V
during the start cycle.
The soft-start cycle should be evaluated to verify that the
reduced R
modulation of the V
If V
the soft-start cycle, converter output ripple should be
16
SS
OUT(SS)
V
R
C
OUT(SS)
C
SS
is typically set to 200k for most applications.
SS
SS
pin has an excessive ripple component during
1
can be set using the following equation:
=
=
) follows the relation:
1 3 10
SS
2 10
. •
Δ
= 220mV + R
•
V
SS
value allows operation without excessive
OUT
V
OUT
pin biases to 220mV during the soft-start
–
–
6
6
•
U
C
SS
t
SS
pin before finalizing the design.
, the signal component caused by
SS
U
SS1
• 2 • 10
, using a value that corre-
SS
SS
= 200k.
SS
W
) is programmed via a
, so the output has to
– 6
OUT
SS1
) will be im-
. The value
U
SS
. With
C
pin
reduced. This is typically accomplished by increasing
output capacitance and/or reducing output capacitor ESR.
External Current Limit Foldback Circuit
An additional startup voltage offset can occur during the
period before the LT3724 soft-start circuit becomes ac-
tive. Before the soft-start circuit throttles back the V
in response to the rising output voltage, current as high as
the peak programmed current limit (I
switched inductor. Switching will stop once the soft-start
circuit takes hold and reduces the voltage on the V
but the output voltage will continue to increase as the
stored energy in the inductor is transferred to the output
capacitor. With I
edge rise on V
follows the relation:
ΔV
OUT
Figure 6. Desirable Soft-Start Characteristic
=
Figure 5. Soft-Start Characteristic
Showing Excessive Ripple Component
I
MAX
OUT
MAX
•
due to energy stored in the inductor
⎛
⎜
⎝
in the inductor, the resulting leading-
TIME, 250μs/DIV
C
TIME, 250 s/DIV
OUT
L
⎞
⎟
⎠
/ 1 2
MAX
3724F05
3724F06
) can flow in the
V
V(V
V
V
V(V
V
OUT
OUT(SS)
OUT
OUT(SS)
C
C
)
)
C
C
3724fb
pin,
pin